In a future 5G high-frequency communication, phase noise caused by a system crystal oscillation may not be ignored. Influences of the phase noise on an orthogonal frequency division multiplexing (OFDM) system mainly include two parts: common phase error (CPE) and inter-channel interference (ICI). In theory, influence of the CPE on every subcarrier of one OFDM symbol is same, which causes a common phase rotation that affects a performance of a receiver. While the influence of the ICI on each subcarrier is different, therefore, a phase tracking reference signal is mainly used for tracking and compensating for the CPE. Since a coherent time of the phase noise is very short and it varies within each OFDM symbol, therefore, a phase noise tracking reference signal (PT-RS) resource pattern is required to be designed densely in time domain, while sparsely in frequency domain. In addition, the PT-RS may further track and compensate for system residual frequency shift and Doppler frequency shift.
Since the user equipment (UE) side has a high demand for a peak-to-average power ratio (PAPR) of a transmitted signal, an additional discrete Fourier transform (DFT) operation is conducted for an uplink DFT-Spread OFDM (DFT-S-OFDM) at a transmitting terminal side compared to conventional OFDM waveform. Referring to FIG. 1, the data in the time-domain from the UE is transformed into the frequency domain by an M-point DFT firstly, and then goes through an N-point inverse fast Fourier transform (IFFT) following a subcarrier mapping, so that the uplink transmitted signal has a single carrier characteristic and the peak-to-average power ratio is decreased. While for the traditional OFDM waveform, the data in the frequency domain is mapped directly to a subcarrier at the transmitting terminal, and then goes through an N-point IFFT once to be transformed into the time domain.
For the PT-RS design of the DFT-S-OFDM, current 3GPP proposals mainly focus on two PT-RS insertion modes: pre-DFT and post-DFT. For the post-DFT mode, there are mainly includes ways: rate matching and puncturing, which both increase the peak-to-average power ratio of the transmitted signal and may increase complexity of computing the DFT and deteriorate performance of the system. In contrast, for the pre-DFT mode, as long as the power of the PT-RS matches with the power of the user data to be sent, the data to be sent via physical uplink shared channel (PUSCH), the peak-to-average power ratio of the transmitted signal and the complexity of computing the DFT does not increase, and the system performance does not decrease either. For the pre-DFT mode, only a single PT-RS in the time domain is currently considered.
Considering that the uplink DFT-S-OFDM waveform is mainly used in a scenario with a limited link quality, a signal-to-noise ratio of the system is not very high, and influence of the random noise on the PT-RS may not be neglected. Therefore, the current design that inserts a single PT-RS in the time domain may result in a wrong phase estimation, which does not improve but deteriorate the system performance.